1. Field of the Invention
This invention relates to a field-effect transistor having a channel formation layer of silicon carbide disposed in contact with source and drain regions, and more particularly, it relates to a silicon carbide field-effect transistor in which the contact formed between the channel formation layer and the source region exhibits different electric characteristics from those of the contact formed between the channel formation layer and the drain region. This invention also relates to a method for the production of such a silicon carbide field-effect transistor.
2. Description of the Prior Art
Silicon carbide (SiC) is a semiconductor material with a wide band gap of 2.2 to 3.3 electron-volts (eV), which is thermally, chemically and mechanically quite stable, and also has a great resistance to radiation damage. Furthermore, the saturation drift velocity of electrons in silicon carbide is greater than that in silicon (Si) and other semiconductor materials. The use of semiconductor devices made of conventional semiconductor materials such as silicon is difficult under severe conditions of high temperature, high output drive, high frequency operation, and radiation exposure. Therefore, semiconductor devices using silicon carbide are expected to have wide applications for devices which are used under such conditions.
For the purpose of fabricating such a silicon carbide semiconductor device, there has been developed a process for growing a large-sized high-quality SiC single crystal on an inexpensive and commercially available Si single-crystal substrate by chemical vapor deposition (Japanese Laid-Open Patent Publication No. 59-203799). Other conventional processes include growing a pure and high-quality SiC bulk single crystal by sublimation (Yu. M. Tairov and V. F. Tsvetkov, J. Crystal Growth 52(1981), p. 146); and growing an SiC single-crystal layer by chemical vapor deposition on an SiC bulk single-crystal substrate obtained by sublimation.
The above-mentioned conventional methods make it possible to control the conductivity type, the impurity concentration, or the like of silicon carbide single crystals by adding appropriate impurities during the growth of the single crystals. Therefore, SiC single crystals prepared by these methods have been widely used for the development of various semiconductor devices. Examples of the silicon carbide semiconductor devices include an inversion-mode metal-oxide-semiconductor field-effect transistor (MOSFET) fabricated on a silicon carbide layer which functions as a channel formation layer (see, for example, U.S. patent application Ser. No. 07/534,046, filed on Jun. 6, 1990; and J. W. Palmour, H. S. Kong, and R. F. Davis, J. Appl. Phys. 64, 2168 (1988)). As used herein, the term channel formation layer refers to a semiconductor layer in which a channel region will be formed in the on state of the transistor.
For the purpose of producing an inversion-mode MOSFET, a channel formation layer should be formed in a semiconductor substrate or semiconductor layer grown thereon. The channel formation layer should also be provided with source and drain regions of the opposite conductivity type thereto. In other words, when an inversion-mode n-channel MOSFET is fabricated with the use of a p-type channel formation layer, the source and drain regions of the n-type must be formed in the p-type channel formation layer. When an inversion-mode p-channel MOSFET is fabricated with the use of an n-type channel formation layer, the source and drain regions of the p-type must be formed in the n-type channel formation layer.
In a field-effect transistor such as described above, the electric characteristics of the contacts formed between the channel formation layer and the source region and between the channel formation layer and the drain region greatly affect the device characteristics of the transistor itself.
In general, a field-effect transistor can be put into practical use in the following conditions of: (1) high breakdown voltage between the source and drain regions; (2) sufficiently reduced leakage current to the substrate when applying a drain voltage; and (3) sufficiently reduced on-state resistance of the transistor.
At present, for the production of silicon carbide field-effect transistors which have been recently developed, source and drain regions are simultaneously formed in a channel formation layer by ion implantation. Thus, the p-n junction formed between the channel formation layer and the source region exhibits the same characteristics as those of the p-n junction formed between the channel formation layer and the drain region. Such a field-effect transistor does not attain excellent device characteristics because it does not satisfy the above-mentioned conditions. In order to satisfy those conditions, the electric characteristics of the p-n junction between the channel formation layer and the source region should be different from those of the p-n junction between the channel formation layer and the drain region, as will be described below.
Among the above-mentioned conditions, the conditions (1) and (2) depend on the electric characteristics (particularly, in the reverse direction) of the p-n junction formed between the channel formation layer and the drain region, while the condition (3) depends on the electric characteristics (particularly, in the forward direction) of the p-n junction formed between the channel formation layer and the source region, and it also depends on the resistance of the source region.
The development of a field-effect transistor having source and drain regions which can attain the above-mentioned respective electric characteristics has not yet been successful. Thus, a silicon carbide field-effect transistor with excellent device characteristics has not yet been put into practical use.